Effect of Cr on Microstructure and Properties of WVTaTiCrx Refractory High-Entropy Alloy Laser Cladding

WVTaTiCrx (x = 0, 0.25, 0.5, 0.75, 1) refractory high-entropy alloy coatings were prepared on a 42-CrMo steel plate using laser cladding. The purpose of this work is to investigate the effect of the Cr content on the microstructure and properties of the WVTaTiCrx coating. The morphologies and phase compositions of five coatings with different Cr contents were comparatively observed. In addition, the hardness and high-temperature oxidation resistance of the coatings were also analyzed. As a result, with the increase in Cr, the coating grains were more refined. All the coating is mainly composed of the BCC solid-solution phase, which promotes the precipitation of the Laves phase with the increase in Cr. The addition of Cr greatly improves the hardness, high-temperature oxidation resistance and corrosion resistance of the coating. The WVTaTiCr (Cr1) exhibited superior mechanical properties, especially in terms of its exceptional hardness, high-temperature oxidation resistance and outstanding corrosion resistance. The average hardness of the WVTaTiCr alloy coating reaches 627.36 HV. After 50 h of high-temperature oxidation, the oxide weight of WVTaTiCr increases by 5.12 mg/cm2, and the oxidation rate is 0.1 mg/(cm2·h). In 3.5 wt% NaCl solution, the corrosion potential of WVTaTiCr is −0.3198 V, and the corrosion rate is 0.161 mm/a.


Introduction
Refractory high-entropy alloys (RHEAs) first appeared in public view in 2010 [1]. Thereafter, they aroused great concern from researchers at home and abroad. Research has proved that RHEAs have excellent mechanical properties at high temperatures, even surpassing the widely used nickel-based alloy [2][3][4][5][6]. RHEAs, with excellent mechanical properties at elevated temperatures, are considered promising high-temperature application materials, which are suitable for complex working conditions such as atomic energy, aerospace, the military industry, advanced nuclear reactors, etc. Previous studies focused on mechanical properties at high-and room-temperatures, but with little research and analysis on high-temperature oxidation resistance [7][8][9]. The research shows that adding the proper amount of Al, Cr, and Si elements into the refractory high-entropy alloy can form a dense oxidation film on the surface of the alloy in a high-temperature environment, thus improving the high-temperature oxidation resistance [10][11][12][13]. Unfortunately, elements Al, Cr, and Si very easily form brittle intermetallic compounds with other refractory elements, resulting in reduced mechanical properties of RHEAs [14][15][16]. As a "candidate" for new high-temperature alloy materials, refractory high-entropy alloys still have top priority in improving high-temperature oxidation resistance.
In recent years, W-based alloys have been widely used in the nuclear energy industry, and W-Ta [17], W-V [18], W-Ti [19], W-Cr [20] have been studied. Research results showed that some W-based binary alloys showed brittle behavior [21]. The reduction in ductility makes it difficult for W-based binary alloys to be widely used in harsh working environments, which also urges us to shift our research focus to W-based high-entropy alloys, This cladding experiment was completed on the LYHS series ultra-high-speed lasercladding machine tool, whose workbench can move the X-Y-Z three axes with multiple degrees of freedom. In this experiment, the presetting method of coating was used, and argon gas was used as a protective gas. After several parameter adjustments, the optimum process parameters were as follows: the laser power was 1.4 kw, the scanning speed was 8 mm/s, the spot diameter was 6 mm, and the overlap rate was 30-40%.
After cladding, the coating surfaces were polished with sandpaper, then corroded with aqua regia (the ratio of concentrated nitric acid to the concentrated hydrochloric acid solution was 1:3), and cleaned with absolute ethanol. A scanning electron microscope (SEM, Phenom ProX, Phenom-World, Eindhoven, The Netherlands) and its accompanying energy dispersive analyzer (EDS) were used to observe and measure the surface morphology, metallographic structure, and chemical composition content of the coatings. The Smart Lab X-ray diffractometer (Rigaku, Tokyo, Japan) (anode Cu target, X-ray Kα, X-ray wavelength 0.1542 nm, and scanning rate ranging from 1 • /min to 20 • /min) was used for phase analysis of the coating.
The hardness changes in the samples were analyzed with HXD-1000TMC microhardness tester under a load of 1.96 N and a loading time of 15 s. Hardness tests were carried out every 0.1 mm along the vertical direction of the interface between the substrate and the cladding layer, and the hardness changes in the coatings were recorded. The original quality of the sample was weighed with an electronic balance (accuracy 0.0001 g) and recorded. The sample was put into the corundum ceramic boat, the ceramic boat was placed in the center of the muffle furnace, and the heating rate set at 5 • C/min and the target temperature at 800 • C. After taking out the sample every 10 h, it was air-cooled quickly, weighed, and the quality change recorded. The whole high-temperature oxidation experiment lasted 50 h. A dynamic polarization electrochemical test in the 3.5 wt% NaCl aqueous solution was conducted with a potentiostat (Gamary Interface 1000, Warminster, PA, USA) to evaluate the corrosion behaviors of the RHEA coatings. The cladding layer was the working electrode, and the auxiliary and reference electrodes were platinum and saturated calomel, respectively. The corrosion current density (I corr ), corrosion potential (E corr ), and corrosion rate (V) were determined using the Tafel analysis method.

Microstructure Characterization
WVTaTiCr x (x = 0, 0.25, 0.5, 0.75, 1) RHEA coatings were prepared under the optimum process parameters, and the cladding condition of the coating cross section was observed for WVTaTi(Cr 0 ) as an example, and the results are shown in Figure 1. From Figure 1a, it is observed that the thickness of the coatings is about 1 mm and the thickness of the heat-affected zone (HAZ) is 0.3~0.5 mm. From Figure 1b,c, it is seen that the metallurgical bonding between the 42CrMo steel and the coating is good, and there are no obvious cracks and pores near the bonding zone (BZ). In addition, the typical dendrite structures are formed near the interface between the coating and the substrate, and grow perpendicular to the boundary.  Figure 2 shows the microscopic morphology of different coatings. It can be seen from the diagram that the structural morphology of the cladding layer changes obviously with the change in element content. As shown in Figure 2a,b, when there is no, or only a small amount of, Cr in the coating, the coating mainly consists of white petal-like dendrites and  Figure 2 shows the microscopic morphology of different coatings. It can be seen from the diagram that the structural morphology of the cladding layer changes obviously with the change in element content. As shown in Figure 2a,b, when there is no, or only a small amount of, Cr in the coating, the coating mainly consists of white petal-like dendrites and a gray base. As the elemental Cr was added to the coating, the dendritic structure became uniform. From Figure 2d, it can be seen that feather-like structures were generated in the coating. In Figure 2e, it is observed that the structure of Cr 1 refines and the area of feather-like tissue increases, showing a network distribution within the dendritic region.  Figure 2 shows the microscopic morphology of different coatings. It can be s the diagram that the structural morphology of the cladding layer changes obvio the change in element content. As shown in Figure 2a,b, when there is no, or onl amount of, Cr in the coating, the coating mainly consists of white petal-like dend a gray base. As the elemental Cr was added to the coating, the dendritic structur uniform. From Figure 2d, it can be seen that feather-like structures were generat coating. In Figure 2e, it is observed that the structure of Cr1 refines and the area o like tissue increases, showing a network distribution within the dendritic region  Figure 3 shows the EDS element distribution of the WVTaTiCr x RHEA coating. The figure shows that during the solidification process of the alloy powder, the distribution of elements in the cladding layer is not uniform. Ta and W with high melting points are first solidified and formed, and mainly distributed in the white dendrite area; there is a high content of V and Cr elements in the gray area, and Ti is uniformly distributed in the alloy. In addition, the minimum enthalpy of mixing (−7 KJ/mol) of Cr and Ta means that during the solidification process, Cr and Ta are more easily combined to form intermetallic compounds, which leads to the formation of Cr 2 Ta in the Laves phase. It is evident from Figure 3e that the feathered Laves phase distribution forms a network with white dendritic areas. In addition, the presence of Cr was also detected in Figure 3a. As the laser cladding makes the 42-CrMo steel dilute, the Cr element in the substrate penetrates the coating. the solidification process, Cr and Ta are more easily combined to form inter compounds, which leads to the formation of Cr2Ta in the Laves phase. It is evid Figure 3e that the feathered Laves phase distribution forms a network wit dendritic areas. In addition, the presence of Cr was also detected in Figure 3a. As cladding makes the 42-CrMo steel dilute, the Cr element in the substrate penet coating.

Phase Analysis
Phase analysis of five high-entropic alloy-coated samples with different con the Cr element was carried out, and the phase in the alloy was relatively simple. T

Phase Analysis
Phase analysis of five high-entropic alloy-coated samples with different contents of the Cr element was carried out, and the phase in the alloy was relatively simple. The main refractory metal elements are the BCC phase, so refractory high-entropy alloys are mostly simple BCC solid-solution phase, and sometimes the second phase is precipitated [35]. As shown in Figure 4, the positions of the diffraction peaks of the four components are approximately the same, consisting of the main peak of one (110) crystal plane and the low peak of (200) (211) crystal planes, mainly concentrated at the diffraction angles of 44.484 • , 64.777 • and 81.983 • . Compared with the PDF card (PDF#34-0396), WVTaTiCr x RHEA coatings are based on the BCC solid-solution phase, and the addition of Cr promotes the precipitation of the Laves phase (Cr 2 Ta), because the addition of Cr causes more severe lattice distortion in the alloy, thus opening a path for atomic diffusion [36]. At the same time, the enthalpy of mixing formed between Cr and other refractory elements is small, which reduces the enthalpy value in the alloy and the stability of the BCC phase, which leads to the precipitation of the Laves phase. From the thermodynamic point of view, if the enthalpy of mixing between the elements is positive, then the atoms of the two elements tend to repel each other, whereas the atoms of the two elements tend to attract each other to form a stable phase. The atomic radius of the Cr atom is the smallest and the easiest to diffuse, and the enthalpy of mixing between Cr and Ta is −7 KJ/mol. The binding ability between Cr and Ta is strong, and it is easier to form Cr 2 Ta, which can be precipitated as the Laves phase. There are no other complex intermetallic compounds in the figure, so the high-entropy effect effectively inhibits the precipitation of intermetallic compounds. more severe lattice distortion in the alloy, thus opening a pa At the same time, the enthalpy of mixing formed between Cr an is small, which reduces the enthalpy value in the alloy and the which leads to the precipitation of the Laves phase. From th view, if the enthalpy of mixing between the elements is positiv elements tend to repel each other, whereas the atoms of the t each other to form a stable phase. The atomic radius of the Cr easiest to diffuse, and the enthalpy of mixing between Cr and ability between Cr and Ta is strong, and it is easier to form Cr2T as the Laves phase. There are no other complex intermetallic c the high-entropy effect effectively inhibits the precipitation of   Figure 5a depicts the hardness distribu high-entropy alloy coatings, and Figure 5b depicts the averag alloy coatings. As can be seen from Figure 5a, the entire crossareas, namely, coating, heat-affected zone (HAZ), and subs whole coating area is significantly higher than that of the subs the laser cladding technology outputs a lot of heat momenta laser beam, and the pre-set coating melts and solidifies rapid supercooling degree during solidification, increases the n shorter growth time for each dendrite, collides with other gr effect of grain refinement is significantly enhanced; thus, the generally higher than that of the substrate. As can be seen increase in Cr content, the average hardness of the coating a previous studies, the hardness value of alloys has a strong rel   Figure 5a depicts the hardness distribution of the cross-section of high-entropy alloy coatings, and Figure 5b depicts the average hardness of high-entropic alloy coatings. As can be seen from Figure 5a, the entire cross-section is divided into three areas, namely, coating, heat-affected zone (HAZ), and substrate. The hardness of the whole coating area is significantly higher than that of the substrate. This is mainly because the laser cladding technology outputs a lot of heat momentarily through a high-energy laser beam, and the pre-set coating melts and solidifies rapidly, which produces a large supercooling degree during solidification, increases the nucleation rate, promotes a shorter growth time for each dendrite, collides with other grains, and grows again. The effect of grain refinement is significantly enhanced; thus, the hardness of the coating is generally higher than that of the substrate. As can be seen from Figure 5b, with the increase in Cr content, the average hardness of the coating also increases. According to previous studies, the hardness value of alloys has a strong relationship with the elements and phase composition. Combining XRD with SEM analysis, with the increase in Cr content, the Laves phase precipitation in the alloy also increases, and the precipitation strengthening effect is enhanced. In addition, with the increase in Cr content, the solid-solution strengthening effect in the alloy is enhanced, which further increases the hardness. solution strengthening effect in the alloy is enhanced, which further increases hardness.  Figure 6 summarizes the trend of oxide weight gain on different component coa surfaces after 50 h continuous oxidation of laser cladding samples at 800 °C. It can be s from Figure 6 that, after adding the Cr element based on WVTaTi (Cr0), the oxide we gain decreases significantly. With the increase in oxidation time, the oxide weight-g curve has continuity, and the content of Cr in the coating will affect the overall h temperature oxidation resistance. In the first half of the experiment, the rate of oxida weight increase is very fast, while, on the contrary, the rate of oxidation weight incre is slow after 20 h. Previous studies have shown that the high content of Cr in high-entr alloys and the slow diffusion effect of high-entropy alloys can improve the h temperature oxidation resistance of high-entropy alloys [12,37,38]. Weight gain (mg/cm 2 ) and weight-gain rate (mg/(cm 2 ·h)) of WVTaTiCrx RH coating after oxidation at 800 °C for 50 h are shown in Table 3. When Cr is not added, weight gain and oxidation rate of the WVTaTi high-entropy alloy coating oxide are 1 mg/cm 2 and 0.21 mg/(cm 2 h), respectively. With the increase in Cr content, the we increases of oxide on the coating surface decreases. The addition of Cr resulted in smallest increase in oxidation weight of the Cr1 RHEA coating, with a mass increas 5.12 mg/cm 2 and an oxidation rate of 0.10 mg/(cm 2 h). According to the oxidat resistance rating standard of steel, superalloy, and high-temperature protective coati (HB5258-2000) [39], it can be seen that Cr1 RHEA coating belongs to the antioxidant all and Cr0.25, Cr0.5, and Cr0.75 belong to the sub-antioxidant alloys. In general, the pro  Figure 6 summarizes the trend of oxide weight gain on different component coating surfaces after 50 h continuous oxidation of laser cladding samples at 800 • C. It can be seen from Figure 6 that, after adding the Cr element based on WVTaTi (Cr 0 ), the oxide weight gain decreases significantly. With the increase in oxidation time, the oxide weight-gain curve has continuity, and the content of Cr in the coating will affect the overall high-temperature oxidation resistance. In the first half of the experiment, the rate of oxidation weight increase is very fast, while, on the contrary, the rate of oxidation weight increase is slow after 20 h. Previous studies have shown that the high content of Cr in high-entropy alloys and the slow diffusion effect of high-entropy alloys can improve the high-temperature oxidation resistance of high-entropy alloys [12,37,38]. solution strengthening effect in the alloy is enhanced, which further increases the hardness.  Figure 6 summarizes the trend of oxide weight gain on different component coating surfaces after 50 h continuous oxidation of laser cladding samples at 800 °C. It can be seen from Figure 6 that, after adding the Cr element based on WVTaTi (Cr0), the oxide weight gain decreases significantly. With the increase in oxidation time, the oxide weight-gain curve has continuity, and the content of Cr in the coating will affect the overall hightemperature oxidation resistance. In the first half of the experiment, the rate of oxidation weight increase is very fast, while, on the contrary, the rate of oxidation weight increase is slow after 20 h. Previous studies have shown that the high content of Cr in high-entropy alloys and the slow diffusion effect of high-entropy alloys can improve the hightemperature oxidation resistance of high-entropy alloys [12,37,38]. Weight gain (mg/cm 2 ) and weight-gain rate (mg/(cm 2 ·h)) of WVTaTiCrx RHEA coating after oxidation at 800 °C for 50 h are shown in Table 3. When Cr is not added, the weight gain and oxidation rate of the WVTaTi high-entropy alloy coating oxide are 10.41 mg/cm 2 and 0.21 mg/(cm 2 h), respectively. With the increase in Cr content, the weight increases of oxide on the coating surface decreases. The addition of Cr resulted in the smallest increase in oxidation weight of the Cr1 RHEA coating, with a mass increase of 5.12 mg/cm 2 and an oxidation rate of 0.10 mg/(cm 2 h). According to the oxidationresistance rating standard of steel, superalloy, and high-temperature protective coatings (HB5258-2000) [39], it can be seen that Cr1 RHEA coating belongs to the antioxidant alloys, and Cr0.25, Cr0.5, and Cr0.75 belong to the sub-antioxidant alloys. In general, the proper addition of the Cr element can effectively improve the oxidation resistance of the coating. Weight gain (mg/cm 2 ) and weight-gain rate (mg/(cm 2 ·h)) of WVTaTiCr x RHEA coating after oxidation at 800 • C for 50 h are shown in Table 3. When Cr is not added, the weight gain and oxidation rate of the WVTaTi high-entropy alloy coating oxide are 10.41 mg/cm 2 and 0.21 mg/(cm 2 ·h), respectively. With the increase in Cr content, the weight increases of oxide on the coating surface decreases. The addition of Cr resulted in the smallest increase in oxidation weight of the Cr 1 RHEA coating, with a mass increase of 5.12 mg/cm 2 and an oxidation rate of 0.10 mg/(cm 2 ·h). According to the oxidationresistance rating standard of steel, superalloy, and high-temperature protective coatings (HB5258-2000) [39], it can be seen that Cr 1 RHEA coating belongs to the antioxidant alloys, and Cr 0.25 , Cr 0.5 , and Cr 0.75 belong to the sub-antioxidant alloys. In general, the proper addition of the Cr element can effectively improve the oxidation resistance of the coating.  Figure 7 shows the surface micromorphology of the coating of WVTaTiCr x RHEA after oxidation at 800 • C for 50 h. As can be seen from the figure, the coating surface generates large areas of irregular polygonal blocky oxides, which are overlapped with different sizes. To intuitively observe the distribution of elements, surface scanning analysis was performed on the selected areas of the oxidized surface, and the results are shown in Figure 8. From the figure, it can be seen that with the high-temperature oxidation in the same environment, the elemental distribution on the surface of the five coatings is approximately the same, and it can also be judged that the oxides on the surface are also basically similar. EDS showed that the oxygen content in the dendrite was greater than the interdendritic region, indicating that the oxidation resistance of the dendrites was weaker than the interdendritic region. V, Cr, Ti, and O are enriched elements, mainly concentrated in interdendritic regions; the proportion of W and Ta content on the entire surface is small. Moreover, as shown in Figure 3a, the dilution of laser cladding technology allows Cr from 42CrMo steel to diffuse into the WVTaTi coating. However, due to the small amount of Cr content, it cannot be detected after high-temperature oxidation.   Figure 7 shows the surface micromorphology of the coating of WVTaTiCrx RHEA after oxidation at 800 °C for 50 h. As can be seen from the figure, the coating surfac generates large areas of irregular polygonal blocky oxides, which are overlapped wit different sizes. To intuitively observe the distribution of elements, surface scannin analysis was performed on the selected areas of the oxidized surface, and the results ar shown in Figure 8. From the figure, it can be seen that with the high-temperature oxidatio in the same environment, the elemental distribution on the surface of the five coatings i approximately the same, and it can also be judged that the oxides on the surface are als basically similar. EDS showed that the oxygen content in the dendrite was greater tha the interdendritic region, indicating that the oxidation resistance of the dendrites wa weaker than the interdendritic region. V, Cr, Ti, and O are enriched elements, mainl concentrated in interdendritic regions; the proportion of W and Ta content on the entir surface is small. Moreover, as shown in Figure 3a, the dilution of laser claddin technology allows Cr from 42CrMo steel to diffuse into the WVTaTi coating. However due to the small amount of Cr content, it cannot be detected after high-temperatur oxidation.  To explore the types of oxides, XRD analysis was performed on the surface of th coatings after oxidation. As shown in Figure 9, the oxide mainly consists of CrTaO4, TiO and Cr2O3; the Cr1 alloy has the highest oxide diffraction peak, indicating that more oxide are generated, making the oxide film on the coating surface uniform and dense. The dens oxide film inhibits the diffusion of oxygen atoms, reduces the oxidation rate, and improve oxidation resistance. This conclusion is also consistent with the results presented by th oxide weight-gain curve. The results show that adding Cr to the WVTaTi alloy ca effectively improve high-temperature oxidation resistance. To explore the types of oxides, XRD analysis was performed on the surface of the coatings after oxidation. As shown in Figure 9, the oxide mainly consists of CrTaO 4 , TiO 2, and Cr 2 O 3 ; the Cr 1 alloy has the highest oxide diffraction peak, indicating that more oxides are generated, making the oxide film on the coating surface uniform and dense. The dense oxide film inhibits the diffusion of oxygen atoms, reduces the oxidation rate, and improves oxidation resistance. This conclusion is also consistent with the results presented by the oxide weight-gain curve. The results show that adding Cr to the WVTaTi alloy can effectively improve high-temperature oxidation resistance.  Table 4 shows the standard Gibbs free energy of molar formation of the oxide Ta, and Ti at 800 °C. Due to the smallest Gibbs free energy of TiO2, it is first generat exists stably. The Gibbs free energy of Ta2O5 is −598 KJ/mol, which is the second sm after the TiO2. Due to the atomic radius of Ta being slightly larger than Cr and diffusion rate is relatively slow. In addition, there is a lattice distortion effect insid entropy alloys, and atomic diffusion needs to overcome varying resistance. Th diffusion effect makes it more difficult for Ta to generate oxides, so Ta2O5 does not on the surface. Although the standard Gibbs free energy of molar formation of C greater than Ta2O5, the diffusion rate of Cr is faster than Ta, which also contribute formation of Cr2O3. Research shows that the Cr and Ta elements have a strong b capacity with oxygen, so a small amount of CrTaO4 can also be generated. W addition of Cr, the internal lattice distortion effect of the WVTaTiCr alloy is the str and the diffusion rate of each atom is the slowest, resulting in a decrease in the b speed with external oxygen elements. This conclusion confirms the oxide XRD p and oxidation kinetic curves. The addition of Cr makes atomic diffusion slow, a oxidation rate and weight-gain decrease, thereby improving the high-temp oxidation resistance of the alloy.

Corrosion Resistance
The Tafel curves of each coating surface in 3.5 wt% NaCl solution are shown in 10. Table 5 summarizes the corrosion potential (Ecorr), corrosion current density (Ico corrosion rate (V) in the Tafel curves of each coating. According to Figure 10, th corrosion potential of the WVTaTi (Cr0) alloy is the lowest, and the self-corrosion po of the alloy gradually increases with the addition of Cr. Among these, the WVTaCr alloy coating has the highest corrosion potential. The data can be derived from T the corrosion potential and corrosion rate of WVTaTi (Cr0) are −0.7783 V and 2.959 respectively, and the corrosion potential and corrosion rate of the WVTaCrTi (Cr coating are −0.3198 V and 0.161 mm/a, respectively. The addition of Cr can prom generation of the passive film on the surface of the alloy coating, while the  Table 4 shows the standard Gibbs free energy of molar formation of the oxides of Cr, Ta, and Ti at 800 • C. Due to the smallest Gibbs free energy of TiO 2 , it is first generated and exists stably. The Gibbs free energy of Ta 2 O 5 is −598 KJ/mol, which is the second smallest, after the TiO 2 . Due to the atomic radius of Ta being slightly larger than Cr and Ti, the diffusion rate is relatively slow. In addition, there is a lattice distortion effect inside high-entropy alloys, and atomic diffusion needs to overcome varying resistance. This slow diffusion effect makes it more difficult for Ta to generate oxides, so Ta 2 O 5 does not appear on the surface. Although the standard Gibbs free energy of molar formation of Cr 2 O 3 is greater than Ta 2 O 5 , the diffusion rate of Cr is faster than Ta, which also contributes to the formation of Cr 2 O 3 . Research shows that the Cr and Ta elements have a strong binding capacity with oxygen, so a small amount of CrTaO 4 can also be generated. With the addition of Cr, the internal lattice distortion effect of the WVTaTiCr alloy is the strongest, and the diffusion rate of each atom is the slowest, resulting in a decrease in the binding speed with external oxygen elements. This conclusion confirms the oxide XRD patterns and oxidation kinetic curves. The addition of Cr makes atomic diffusion slow, and the oxidation rate and weight-gain decrease, thereby improving the high-temperature oxidation resistance of the alloy.

Corrosion Resistance
The Tafel curves of each coating surface in 3.5 wt% NaCl solution are shown in Figure 10. Table 5 summarizes the corrosion potential (E corr ), corrosion current density (I corr ), and corrosion rate (V) in the Tafel curves of each coating. According to Figure 10, the self-corrosion potential of the WVTaTi (Cr 0 ) alloy is the lowest, and the self-corrosion potential of the alloy gradually increases with the addition of Cr. Among these, the WVTaCrTi (Cr 1 ) alloy coating has the highest corrosion potential. The data can be derived from Table 5; the corrosion potential and corrosion rate of WVTaTi (Cr 0 ) are −0.7783 V and 2.959 mm/a, respectively, and the corrosion potential and corrosion rate of the WVTaCrTi (Cr 1 ) alloy coating are −0.3198 V and 0.161 mm/a, respectively. The addition of Cr can promote the generation of the passive film on the surface of the alloy coating, while the oxide deposition of Cr can fill the vacancy of the passive film, making the surface passive film become denser and uniform, thus reducing the diffusion and penetration rate of ions in the NaCl solution, decreasing the corrosion rate and improving the corrosion resistance of the alloy surface coating.

Conclusions
The WVTaTiCrx (x = 0, 0.25, 0.5, 0.75, 1) RHEA coating was prepared using cladding on the 42-CrMo steel surface. The effect of the Cr content on the microstru and mechanical properties of the WVTaTiCrx (x = 0, 0.25, 0.5, 0.75, 1) alloy was stu The analysis results are as follows: (1) Laser-cladding-forming figures showed good metallurgical bonding betwee coatings and the substrate, a uniform and dense coating structure, and a co thickness of about 1 mm. The results indicate that laser cladding is feasible fo preparation of RHEA coatings. (2) The WVTaTi (Cr0) alloy coating consists of the BCC phase, and the addition promotes the precipitation of the Laves, which is uniformly distributed in structure. The WVTaTiCr (Cr1) exhibits superior mechanical properties, particu in terms of its exceptional hardness, high-temperature oxidation resistance outstanding corrosion resistance.

Conclusions
The WVTaTiCr x (x = 0, 0.25, 0.5, 0.75, 1) RHEA coating was prepared using laser cladding on the 42-CrMo steel surface. The effect of the Cr content on the microstructure and mechanical properties of the WVTaTiCr x (x = 0, 0.25, 0.5, 0.75, 1) alloy was studied. The analysis results are as follows: (1) Laser-cladding-forming figures showed good metallurgical bonding between the coatings and the substrate, a uniform and dense coating structure, and a coating thickness of about 1 mm. The results indicate that laser cladding is feasible for the preparation of RHEA coatings. (2) The WVTaTi (Cr 0 ) alloy coating consists of the BCC phase, and the addition of Cr promotes the precipitation of the Laves, which is uniformly distributed in the structure. The WVTaTiCr (Cr 1 ) exhibits superior mechanical properties, particularly in terms of its exceptional hardness, high-temperature oxidation resistance, and outstanding corrosion resistance. (3) The successful preparation of WVTaTiCr alloy coating compensates for the performance shortcomings of some W-based binary alloys, providing a new idea for the development of heat insulation and radiation protection materials for nuclear reactors. The improved mechanical and oxidation properties of WVTaTiCr indicate the potential for future use in fusion applications.
Author Contributions: Conceptualization, Z.S. and Z.W.; experimental design, material preparation, and data collection, Z.X. and C.L.; writing-original draft preparation, Z.X.; writing-review and editing, Z.S.; formal analysis, Z.X.; investigation, C.L.; supervision, Z.S. and Z.W. All authors have read and agreed to the published version of the manuscript.
Funding: This study was supported by the Shandong Provincial Science and Technology Development Plan Project (grant number 2014GGX103009).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.